Was learning to lead trad on a multi-pitch route, and didn't set up anchors correctly.  I was belaying off myself (not the anchor), and had a ton of slack between myself and the anchor.  The follower pulled out the last piece of pro and then fell.  Since I had all this slack behind me, I went rolling down the rock until all the slack was out between myself and anchor.  Luckily, follower was fine, but felt like absolute moron!!